In trying to understand animal communication, it is essential to know about what the animals are communicating. Traditionally resource-dividing behavior patterns have been viewed as agonistic or aggressive acts in which one animal gains resources at the expense of the other. Analysis of hermit crab shell-related interactions has indicated that frequently both individuals gain (in shell fit) from the exchange of shells. Thus the communication preceeding an exchange could be viewed as negotiation, rather than aggression. The proposed study would look for evidence of mutual gain in interspecific interactions in three species of hermit crabs. Gain would be measured as reduction in the deviation from a preferred shell size by an exchange. It is predicted that in those interactions in which an exchange does occur, the gain will be greater than in those interactions terminated without an exchange of shells.